Subsea navigation and survey

ABSTRACT

A subsea navigation system comprising a plurality of spaced antenna means provided on a subsea structure arranged to emit electromagnetic radiation. A detection means is provided on a subsea vehicle for receiving and analyzing the electromagnetic radiation transmitted by the antenna means allowing the subsea vehicle to navigate relative to the subsea structure. Each of the antenna means comprises a cathodic protection anode provided on the subsea structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to subsea navigation, for example following ortracking of subsea structures to facilitate inspection or otheroperations, it further relates to surveying the position of subseastructures.

2. Description of Related Art

Performing periodic inspection and/or maintenance is essential in theoperation of subsea pipeline systems. There are two main ways in whichthis is currently conducted, i.e. by the use autonomous underwatervehicles (AUVs) or directly by divers.

The use of AUV's is preferred but problems still arise. For example,when carrying out an inspection, the AUV must be made to follow theappropriate subsea structure closely enough so that it can provideuseful information. Typically, when inspections are being carried out,the AUV will include sonar scanning equipment to form images of thestructure. With such equipment there is a trade off between resolutionand field of view. Therefore, if the precise position of the structureis not known relative to the AUV, the equipment must be set up with arelatively large field of view and a correspondingly low resolution toensure an image of the structure is obtained. In most circumstances, itis either undesirable or impossible for a human operator to monitor theposition of an underwater vehicle relative to the subsea structure andhence AUV's are used which attempt to follow the structure autonomously.

Existing systems use inertial navigation equipment which does notoperate perfectly. Thus the further an AUV is from its start point, thefurther off course it may have wandered. In practice this means that thearray of sonar detectors provided on the AUV has to be configured togive a field of view sufficient to cope with the maximum deviation fromcourse which might arise during the length of the scan. As mentionedabove this leads to a reduction in resolution.

BRIEF SUMMARY OF THE INVENTION

It is an object of a main aspect of this invention to provide anavigation system which allows navigation relative to a subseastructure.

It is an object of another aspect of this invention to provide a subseasurvey method.

According to a first aspect of the invention there is provided a methodof subsea navigation comprising the steps of:

-   causing a plurality of spaced antenna means provided on a subsea    structure to emit electromagnetic radiation; and-   receiving and analyzing said radiation at a subsea vehicle to allow    the subsea vehicle to navigate relative to the subsea structure.

According to a second aspect of the invention there is provided a subseanavigation system comprising a plurality of spaced antenna meansprovided on a subsea structure and arranged to emit electromagneticradiation, and detection means provided on a subsea vehicle forreceiving and analyzing electromagnetic radiation transmitted by theantenna means to allow a subsea vehicle to navigate relative to thesubsea structure.

According to a third aspect of the invention there is provided apparatusfor a subsea navigation system, the apparatus comprising a plurality ofantenna means arranged to be mounted in a spaced relation on a subseastructure for emission of electromagnetic radiation, and detection meansarranged for location on a subsea vehicle for receiving and analyzingelectromagnetic radiation transmitted by the antenna means to allow thesubsea vehicle to navigate relative to the subsea structure.

The term subsea is used in this application as this is conventionalterminology, however it will be appreciated that the term subsea shouldbe taken to include any underwater situation.

In some cases a common signal generating means may be provided fordriving some or all of the antenna means. Preferably however,independent signal generating means are provided for each antenna means.Said apparatus may comprise signal generating means.

The system is typically arranged so that radiation from each antennameans can be received mutually independently to reduce or eliminateinterference or confusion. This may be achieved in a number of differentways.

In some embodiments the radiation emitted by the individual antennameans is not mutually identical. It is particularly preferred if theradiation emitted by one antenna means is detectably different from thatemitted by each adjacent antenna means. Using different frequencies is apreferred way to provide detectably different radiation.

In some embodiments three detectably different instances, typicallythree different frequencies, of radiation may be emitted.

The radiation may be emitted at differing times by each antenna means toaid or allow discrimination.

The system, apparatus or method may be arranged so that the emission ofradiation from the antenna means is triggered by a signal issued fromtransmission means on the subsea vehicle. Such triggering may beselective.

The receiving means and analysing means may be arranged to perform 3axis electric field detection. This allows the direction of a vectortowards the source transmitting the radiation to be found.

The method, system and apparatus may be arranged to allow the subseavehicle to track the subsea structure.

This invention is particularly applicable where the structure to betracked is a pipeline system. In such cases the subsea vehicle willnormally be an Autonomous Underwater Vehicle (AUV) which is being usedto inspect or otherwise interact with the pipeline system.

The method, system and apparatus may be arranged such that theunderwater vehicle is able to determine its position relative to thesubsea structure. In some circumstances this would enable the underwatervehicle to determine its position absolutely relative to the Earth'ssurface. The analyzing means may be arranged to determine the positionof the underwater vehicle relative to the subsea structure. This may beachieved using the results of 3 axis field detection and triangulationtechniques.

A pipeline system typically comprises metallic structure which isprovided with cathodic protection anodes. In such cases, the pluralityof antenna means of the present method, system and apparatus maycomprise a plurality of anodes, each anode acting as or as part of arespective antenna means. Anodes are located at convenient spacedlocations along the structure, can function well as antennas, and, ofcourse, are already present for other reasons. This makes the use ofanodes in the antenna means particularly attractive.

The metallic structure of a pipeline system will generally comprise oneor more flowline and possibly other structures such as manifolds, trees,and wellheads.

According to another aspect of the present invention there is provided amethod for surveying the position of a subsea structure comprising thesteps of:

-   monitoring the position of a subsea vehicle relative to a known    reference point;-   causing a plurality of spaced antenna means provided on the subsea    structure to emit electromagnetic radiation;-   causing the subsea vehicle to travel along the subsea structure; and-   analyzing said radiation as received at the subsea vehicle as it    travels to allow the determination of the position of the subsea    structure relative to the subsea vehicle and hence relative to said    reference point.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 schematically shows part of a pipeline system and a nearby AUV;

FIG. 2 schematically shows an anode and a section of flowline in thepipeline system of FIG. 1; and

FIG. 3 schematically shows a three axis electric field detecting meansprovided at the AUV shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows part of a pipeline system comprising a flowline 1 and awellhead 2 as well as a nearby AUV 3 which is being used to inspect thepipeline system. The AUV 3 comprises appropriate sonar and/or otherscanning equipment SC to allow the inspection and recording of thecondition of the pipeline system.

The pipeline system is a metallic structure and is provided with aplurality of cathodic protection anodes 4 at spaced locations. Referringto FIG. 2, each anode 4 is connected to its respective portion ofmetallic structure 1, 2 via inductance means 5. Further, a respectivetransmitting means 6 is connected across each inductance means 5. Theinductance means 5 is chosen to have virtually zero impedance to thecathodic protection currents which must pass through the anode, whilsthaving a high impedance to signals generated by the respectivetransmitting means 6. In this way it is possible for each anode 4 to actas an antenna when the transmitting means 6 is operated.

In operation, the metallic structure 1, 2 and earth, via the respectiveanode 4 and other remote earthing locations, act as a signal circuit.However, in the present application all that is important is that it ispossible for the anode to radiate electromagnetic waves away from it andhence away from the metallic structure 1, 2.

The AUV 3 comprises detecting means 7 for receiving and analyzing theelectric field caused by electromagnetic radiation emitted from theanodes 4.

The AUV 3 further comprises transmitting means 8 arranged to transmittrigger signals to receiving and control means 9 associated with eachanode 4. Each receiving and control means 9 is arranged to causeoperation of the respective transmitting means 6 upon receipt of anappropriate trigger signal from a nearby AUV 3.

In operation, the AUV 3 begins its inspection journey at a point nearbythe pipeline system. The transmitting means 8 on the AUV 3 is caused toemit a trigger signal which is received by the receiving and controlmeans 9 associated with nearby anodes 4. On receipt of this triggersignal, each receiving and control means 9 causes its respectivetransmitter means 6 to operate and thus cause its respective anode 4 toemit electromagnetic radiation.

It is important that the AUV 3 is able to receive signals from theindividual anodes independently. Thus, some means must be provided toallow the signals to be distinguished. There are a number of ways inwhich this can be accomplished as will be evident to those skilled inthe art.

In the present embodiment, the transmitting means 6 do not all transmitat the same frequency. In particular, three different frequencies f₁,f₂, f₃ are used. The transmitting means 6 are arranged so that eachanode 4 emits radiation at a different frequency than its immediatelyadjacent neighbours.

The detecting means 7 in the AUV 3 receives and analyzes the radiationemitted from any nearby anodes 4 which are transmitting. The detectingmeans 7 comprises a three axis electric field detection meansschematically shown in FIG. 3. The three axis detection means comprisesa non-conductive frame 70 having three mutually perpendicular legs. Acommon electrode 71 is provided at the joining point of the legs andrespective other electrodes 72, 73, 74 and provided at the end of eachleg. A shielded conductor (not shown) is provided from each electrode71-74 to the remainder of the detecting means 7. In use the relativepotential between the common electrode 71 and each of the otherelectrodes 72-73 is measured in turn. As the legs are mutuallyorthogonal these measurements are representative of the x, y, zcomponents of the detected field in the frame of reference determined bythe legs and thus the direction towards the transmitting anode can bedetermined. Preferably the 3 axis detection means is allowed to floatfreely in the sea or is formed around the AUV 3. In one particulararrangement the AUV 3 is housed in a carbon fiber/KEVLAR® cage and partsof the cage act as the legs 70 between electrodes 71-74 provided on itscorners.

The AUV 3 is arranged to use the determined directional information tocontrol the path of the AUV 3 ensuring that it closely tracks thepipeline system and thus that the pipeline system remains within thefield of view of the scanning equipment SC.

In the present embodiment the instantaneous position of the AUV 3relative to the subsea structure is determined using standardtrigonometric techniques. Although this is not essential for simpletracking, it gives greater flexibility in general navigation. Thus thepresent system can be used for general navigation purposes in the regionof the structure.

The AUV 3 may have access to a map of the structure being followed orother data which can be used in determining a new heading. Suchinformation may also be used to allow navigation relative to the Earth'ssurface when the position of the structure is known.

At a straight section of a pipeline, information concerning thedirections to transmitting anodes may be insufficient to pinpoint theAUV 3 relative to the pipeline 1. For this reason a gravity sensor orvertical gyro may be used to provide additional information.

It is possible for the signals emitted by the anodes 4 to carry codedinformation which can be interpreted by the AUV 3. For example the codedinformation may include a unique address or location information toenable the AUV 3 to pinpoint its position along the pipeline structure.Other information, for example sensor readings, may be coded onto thesignals emitted by the anodes 4. This clearly facilitates the collectionof data from components and sensors associated with the subsea structurein addition to, or in alternative to, data/images obtained from thescanning equipment provided on the AUV 3.

Although not described in detail it is also possible for the AUV 3 tosend signals to the pipeline 1, these might then be transmitted alongthe pipeline to a remote location.

The present embodiment makes use of electric signals of frequency up to10 KHz. With such a system, the electric signals transmitted from theanodes 4 should have a range in the order of low hundreds of meters. Inalternatives a two tier frequency regime can be used. Since lowfrequencies will travel further in water, a lower frequency might beused as an AUV 3 homes in, and once close to the pipeline a higherfrequency could be used.

In some implementations, direct, and possibly real time, communicationof the data obtained and processed by the AUV 3 may be made to a surfacevessel. Such communication may be implemented in a number of differentways but preferably the AUV 3 has a tetherless mode of operation so anycommunication to the surface is preferably wireless.

It should be noted that this navigation system is not restricted to usewith AUVs inspecting pipeline systems. On the contrary, it may be usedwith any subsea vehicle where there is a desire to navigate relative toa subsea structure.

In a development of the above system the AUV 3 and transmitting anodes 4may be used to “survey in” a pipeline 1. In this case the position ofthe pipeline relative to the Earth's surface is unknown and is to bedetermined. The system described above is used to record the relativeposition of the pipeline 1 and the AUV 3 as the AUV 3 is moved along thepipeline 1. The position of the AUV 3 as it moves relative to Earth ismonitored using sonar and GPS for example. The combination of knowingthe “absolute” position of the AUV 3 and the position of the AUV 3relative to the pipeline 1 enables the “absolute” position of thepipeline to be mapped.

1. A subsea navigation system comprising a plurality of spaced antennasprovided on a subsea structure and arranged to emit electromagneticradiation, and a detector provided on a subsea vehicle for receiving andanalyzing electromagnetic radiation transmitted by the antennas to allowthe subsea vehicle to navigate relative to the subsea structure, whereineach antenna comprises a respective cathodic protection anode providedon the subsea structure.
 2. A subsea navigation system according toclaim 1 in which an independent signal generator is provided for drivingeach antenna.
 3. A subsea navigation system according to claim 1 whichis arranged so that radiation from each antenna is receivablesubstantially mutually independently.
 4. A subsea navigation systemaccording to claim 3 in which the radiation emitted by one antenna isdetectably different from that emitted by each adjacent antenna.
 5. Asubsea navigation system according to claim 1 which is arranged so thatthe emission of radiation from each antenna is triggered by a signalissued from a transmitter on the subsea vehicle.
 6. A subsea navigationsystem according to claim 5 in which the triggering is selective so asto actuate only at least one selected antenna.
 7. A subsea navigationsystem according to claim 1 in which the detector is arranged to perform3 axis electric field detection enabling determination of the directionof a vector towards the source transmitting the radiation.
 8. A subseanavigation system according to claim 7 in which the detector comprises athree axis detector comprising a non-conductive frame, a commonelectrode and three other electrodes spaced from the common electrode inrespective mutually perpendicular directions.
 9. A subsea navigationsystem according to claim 8 in which the frame has three mutuallyperpendicular legs, the common electrode is provided at the joiningpoint of the legs and each other electrode is provided at the end of arespective leg.
 10. A subsea navigation system according to claim 8 inwhich the subsea vehicle is housed in a cage made at least of carbonfiber and parts of the cage act as the non-conductive frame.
 11. Amethod of subsea navigation comprising the steps of: causing a pluralityof spaced antennas provided on a subsea structure to emitelectromagnetic radiation, each antenna comprising a respective cathodicprotection anode provided on the subsea structure; and receiving andanalyzing said radiation at a subsea vehicle to allow the subsea vehicleto navigate relative to the subsea structure.
 12. Apparatus for a subseanavigation system, the apparatus comprising a plurality of antennasarranged to be mounted in a spaced relation on a subsea structure foremission of electromagnetic radiation, and a detector arranged forlocation on a subsea vehicle for receiving and analyzing electromagneticradiation transmitted by the antennas to allow the subsea vehicle tonavigate relative to the subsea structure, wherein each antennacomprises a respective cathodic protection anode.
 13. A system accordingto claim 1 which allows the subsea vehicle to track the subseastructure.
 14. A method for surveying the position of a subsea structurecomprising the steps of: monitoring the position of a subsea vehiclerelative to a known reference point; causing a plurality of spacedantennas provided on the subsea structure to emit electromagneticradiation, each antenna comprising a respective cathodic protectionanode; causing the subsea vehicle to travel along the subsea structure;and analyzing said radiation as received at the subsea vehicle as ittravels to allow the determination of the position of the subseastructure relative to the subsea vehicle and hence relative to saidreference point.
 15. A method according to claim 11 which allows thesubsea vehicle to track the subsea structure.
 16. Apparatus according toclaim 12 which allows the subsea vehicle to track the subsea structure.17. A subsea navigation system comprising a plurality of spaced antennameans provided on a subsea structure and arranged to emitelectromagnetic radiation, and detection means provided on a subseavehicle for receiving and analyzing electromagnetic radiationtransmitted by the antenna means to allow the subsea vehicle to navigaterelative to the subsea structure, wherein each antenna means comprises arespective cathodic protection anode provided on the subsea structure.18. A method of subsea navigation comprising the steps of: causing aplurality of spaced antenna means provided on a subsea structure to emitelectromagnetic radiation, each antenna means comprising a respectivecathodic protection anode provided on the subsea structure; andreceiving and analyzing said radiation at a subsea vehicle to allow thesubsea vehicle to navigate relative to the subsea structure.